The development of high energy cell systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly active anode materials, such as lithium, calcium, sodium and the like, and the efficient use of high energy density cathode materials, such as FeS.sub.2, SOCl.sub.2, Co.sub.3 O.sub.4, PbO.sub.2 and the like. The use of aqueous electrolytes is precluded in these systems, since the anode materials are sufficiently active to chemically react with water. Therefore, in order to realize the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, it is necessary to use a nonaqueous electrolyte system.
Many cell or battery applications, such as hearing aids, cameras, games, watches, calculators, and the like, require a substantially unipotential discharge source for proper operation. However, it has been found that in many nonaqueous cells employing positive active materials which include a conductive additive such as graphite and/or carbon, the cell upon initially being discharged, exhibits a high voltage whereupon the cell then proceeds to reach its lower operative discharge voltage level only after a certain time period has elapsed. The time period for the cell to reach its intended operating discharge voltage level usually depends on the discharge rate through the load and thus, depending on the apparatus it is to power, could result in a period extending up to several hours or even days. This phenomenon has serious drawbacks when a cell is intended to be used in electronic devices requiring a substantially unipotential discharge source for proper operation. In some of these electronic devices, any initial voltage peak substantially exceeding the intended operating volatage for the device could result in serious damage to the electronic components of the device. One approach to protect devices from batteries exhibiting high voltages prior to leveling off to their desired operating voltage level is to add additional electronic circuit components to protect the main operating components of the device. However, this not only adds to the expense of the device but also would result in enlarging the device to accommodate the protective circuitry. With the emphasis placed on miniaturization, it has become necessary for the battery industry to design smaller and smaller miniature power cells.
It has been found through experience that certain cathode formulations (such as FeS, CuS, Bi.sub.2 S.sub.3, CdO, etc.) contain traces of high voltage impurities that oftentimes lead to the undesirably high initial open circuit voltages. In order to stabilize (that is, reduce) the voltage in new cells, battery manufacturers sometimes resort to a method called "burning in" in which the cell is purposely discharged a predetermined amount before shipment.
In particular, in the lithium-copper sulfide system (Li/CuS), the copper sulfide cathode is made by: (1) blending Cu.sup.o powder, S.sup.o powder, CuS and C; (2) compressing the mixture into pellet form; and (3) sintering the pellet at about 250.degree. C., so as to combine the Cu.sup.o and S.sup.o to form CuS into a cemented cathode of sufficiently high strength to resist breaking. The Li/CuS couple is ideally designed to have an open circuit voltage (OCV) of 2.15 volts per cell but new cells exhibit an excess OCV in the range of about 2.3 to 2.5 volts. As was alluded to earlier, this is undesirable, since excess voltage may be injurious to a device utilizing the cell. Furthermore, it is difficult to utilize OCV as a quality control criterion if it is not uniform.
Heretofore, in order to reduce the excess OCV to a desirable 2.15 volts, the cells would be subject to a burn in amounting to no more than a few percent of the cell's ampere-hour capacity which, in fact, does bring the OCV to 2.15 volts (e.g., one ohm discharge for one minute). Apparently, there is a higher (cathodic) potential specie on the cathode particle surface and the brief discharge reduces it so as not to interfere with the desired CuS potential.
It should be acknowledged that this undesirable OCV phenomenon is not limited to Li/CuS systems. Rather, it occurs with most lithium based systems.